Medical catheters are used for innumerable minimally invasive medical procedures. Catheters may be used, for example, for delivery of therapeutic drug doses to target tissue and/or for delivery of medical devices such as lumen-reinforcing or drug-eluting stents. Likewise, catheters may be used to guide medical instruments to a target site to perform a surgical procedure, such as tissue rescission, ablation of obstructive deposits or myocardial revascularization.
Currently, catheter-based systems that are equipped with sensors (for example, electrodes) have a sensor tip at the distal end of the catheter. The sensor tip may or may not have an opening to permit a needle or a medical device to pass through the opening and into target tissue in the patient. These systems usually have one or more additional return sensors implemented as bands circumferentially around the catheter. In some systems, tissue contact is determined by measuring the impedance between the tip sensor when it is in contact with tissue and a return sensor that is not in contact with the tissue but is only in contact with a fluid, for example, blood, that is surrounding the tissue. However, this determination is based on known, that is, pre-determined, impedance values when the electrode is in contact with tissue and when only in contact with body fluids (for example, blood). Unfortunately, this method is not without problems, since the impedances of tissue and body fluid (for example, blood) are known to change during a procedure and different tissue will have different impedances depending on whether the tissue is healthy or diseased.
Alternatively, other sensor systems using a tip sensor and one or more band sensors around the catheter determine tissue contact by measuring impedance between sensors when the sensors are both contacting the tissue. Consequently, for this type of catheter to be able to detect the impedance, it must be lying flat against the target tissue area so that at least two of the sensors are in contact with the tissue. Unfortunately, this position does not enable the optimal targeted delivery of therapeutic agents to provide the most effective treatment regimen, since the tissue determined to be the target is usually not directly in line with the tip of the catheter.